1. Field of the Invention
The present invention is directed to a gradient coil assembly for an NMR tomography apparatus, the assembly being of the type having at least one gradient coil and one cooling structure, as well as to a method for producing such a gradient coil assembly.
2. Description of the Prior Art
In the operation of an NMR tomography apparatus, for image creation it is necessary to superimpose at least one magnetic field gradient onto a basic magnetic field. This occurs by means of a gradient coil assembly arranged in the basic field. The gradient coil arrangement typically includes a number of coils in order to create three magnetic field gradients residing perpendicularly relative to one another. In the actively shielded gradient coils frequently used today, in addition to the primary coils which create the useful field, secondary coils must be present which prevent the creation of eddy currents in metallic parts surrounding the gradient coil. These secondary coils are arranged concentrically about the primary coils and are at a distance therefrom. Typically, each gradient coil is permeated by a pulse-like current on the order of magnitude of about 300 A. Due to the ohmic resistance of the coil, a considerable amount of energy is converted into heat.
This heat must be conveyed away in a suitable manner. In an NMR tomography apparatus for solid-state examinations, a good cooling of the gradient coils is particularly necessary, because the patient lies in the interior space of the gradient coil assembly, which is constructed in a tubular shape. Excessive heating of this assembly would be uncomfortable for the patient and would compromise the willingness of patients to undergo NMR tomographic examinations.
German Patentschrift 41 41 514 teaches a gradient coil assembly of the above type. Ventilation channels serve as cooling means, these channels being formed in the assembly by U-shaped profiles through the interior space.
In this gradient coil assembly, however, the incorporation of the channels for the cooling medium into the assembly is costly, and the heat conveyance is not optimal.
German OS 34 04 457 teaches an arrangement for cooling a magnet system, this arrangement being provided particularly for attachment to a basic field coil of an NMR tomography apparatus. The cooling includes a number of cooling elements with a heat conductor plate in the form of a ring wheel, this plate being provided with a number of radial slots distributed uniformly around the circumference of the plate. A cooling agent conduit is soldered or welded onto the heat conduit plate or embedded or pressed or cemented into the heat conduit plate.
Such a cooling structure is only suitable for attachment to level surfaces which are to be cooled.
German OS 40 17 260 discloses a method for the production of a gradient coil arrangement of an MRI apparatus.